A touch pad is usually defined as a touch-sensitive user interface area in an electronic device, which allows a user to input information or a command to the device by pressing the touch sensitive area. The touch pad can be used as a keypad having a designated functionality. For example, a touch pad can be used as an on/off switch and the user can turn the electronic device on or off by pressing the touch sensitive area. The touch pad can have several selectable functions. For example, the touch pad can be used as a keyboard having a plurality of soft-keys of different functions to allow the user to enter or select one function at a time.
In some touch pads, it is required to press the pad surface in order to deform it. In such a touch pad, several layers of material, separated by thin spacers, are used to form a grid of vertical and horizontal rows of electrodes. An electrical current is maintained in the grid of electrodes. When a user presses the pad, the layers are caused to make contact with each other at the pressing point, thereby interrupting the current in the electrode grid. A detection circuit is used to detect the interruption in the current and determine the location of the pressing point on the pad. In other touch pads, mere touching of the pad surface by a finger is sufficient. This latter type of touch pad can be of a resistive-type or capacitive-type. On a resistive-type touch pad, a thin, electrically conductive and resistive layer is coated on the surface of the touch pad area. On a capacitive-type touch pad, a coated layer forming a matrix of pixel-like capacitors is provided on the touch sensitive area. When a finger touches the surface, it changes the electrical characteristics of the coated layer. By measuring the resistance or capacitance values at a number of surface points corresponding to the pressing point, the location of the pressing point can be determined. On an inductive-type touch pad, inductive elements are distributed over the touch pad area. A stylus made of an inductive material is used to change the signals transmitted through the inductive elements for the detection of the presence of the stylus. In some touch pads, if the user moves the touching object around the touch sensitive area and a series of pressing points are registered, the movement of the touching object can also be calculated.
When the touch pad is also used as an information display or an image display, the electrodes positioned on top of a touch pad may affect the optical transparency of display and/or distort the displayed text or image. Furthermore, some touch pads can be damaged by having dirt or scratches on the touch pad surface. In recent years, surface wave technology is also used in touch pad applications. In particular, ultrasonic waves propagating on the touch pad surface are used to detect the interruption or absorption of the waves due to touching. While a touch pad using this surface wave technology does not affect the optical quality of the display, it is costly to produce.
It is advantageous and desirable to provide a touch pad, which is cost effective and does not affect its optical quality.